fix(preview): Ensure the protocol is correct in redirects from server actions during preview mode

[sommeeeer]

Closes #686.

Important to note this didn't only affect next-auth. All redirects from server actions was affected in preview mode. wrangler dev would hang and give an error as it would try to send a request to https.

Through my research I figured out that Next internally uses process.env.__NEXT_PRIVATE_ORIGIN in standalone mode. They set it during startServer on this line. To align with this behavior in preview mode this should be set to the origin during the initialization.

Affected lines in Next:

• https://github.com/vercel/next.js/blob/6b1e48080e896e0d44a05fe009cb79d2d3f91774/packages/next/src/server/app-render/action-handler.ts#L177-L184
• https://github.com/vercel/next.js/blob/6b1e48080e896e0d44a05fe009cb79d2d3f91774/packages/next/src/server/app-render/action-handler.ts#L307-L308

proto would always be https here, thus the fetch below would recieve the wrong protocol in its fetchUrl.

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[vicb]

Thanks for the fix!

Could we drop the condition url.hostname === "localhost" and always set process.env.__NEXT_PRIVATE_ORIGIN?

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commit: ad79fcb

[sommeeeer]

Could we drop the condition url.hostname === "localhost" and always set process.env.__NEXT_PRIVATE_ORIGIN?

In theory that shouldn't be a problem. fetchUrl computed by Next would be correct if the protocol is https. This fix is only necessary when the protocol is http. Its up to you.

[vicb]

LGTM, thanks!

Thanks for both the fix and the detailed PR description.

I have updated the code to unconditionally apply the fix.